Of all the thermoplastics manufactured today, probably the most versatile and most widely used class of materials is polymerized monovinyl aromatic compounds such as polystyrene, polymerized alpha-methyl styrene, and polymers of ring-substituted styrenes.
Virgin polystyrene manufactured by the polymerization of styrene monomer often requires the inclusion therein of additives such as pigments, stabilizers, anti-foaming agents, mold-release agents, plasticizers, and anti-oxidants. Plasticizers such as mineral oil and mold-release and stabilizer agents such as zinc stearate are necessary in the polymer to allow it to be formed in thermoforming equipment into the final consumer products. Anti-oxidants such as Irganox 1076, a hindered phenol manufactured by Ciba-Geigy Corporation of Greensboro, N.C., are necessary to prevent the polymer from degrading with age and from exposure to ultra violet light from sources such as sunlight and artificial lighting.
As already mentioned, one of the most desirable, if not the most desirable, lubricant and mold-release agents added to polystyrene and other polymerized monovinyl aromatic compounds is zinc stearate. In conventional polymerization systems, zinc stearate is added to the process by first melting it in a closed heated vessel at 120.degree. to 130.degree. C. and then pumping it into the polymerization system at the desired location. The problems with this approach are many.
First, the molten zinc stearates, as well as other additives, oxidize easily at temperatures above their melting points, and must be completely shut off from any traces of air to prevent oxidation of the material, which causes yellow discoloration of the finished polymer. This is normally achieved by maintaining the headspace in the melting vessel filled with nitrogen.
Second, feeding problems often occur when trying to transport molten zinc stearate to the polymerization system. If any traces of air were allowed to leak in through the lines or fittings to the melt, the afore-mentioned oxidation will occur. Also, if the stearate supply piping were not maintained above the melt temperature of the stearate, the material would begin to solidify and precipitate out, clogging the lines or allowing solid chunks of the material into the polymerization process, adulterating the finished polymer.